Beam self-cleaning for spatiotemporal mode-locked fiber lasers

Mode locking (ML) is the most important technique to generate ultrashort laser pulses. Tremendous progress has been made in locking longitudinal modes in lasers with a single transverse mode, e.g., single-mode fiber lasers. Spatiotemporal mode-locking (STML) has been proposed by locking transverse and longitudinal modes in multimode fiber (MMF) lasers to generate ultrafast spatiotemporal pulses, opening new directions in studies of nonlinear wave propagation. Thanks to fiber multimodality, such lasers generate pulses with higher power with respect to their single-mode counterpart. On the other hand, the irregular beating among transverse modes is detrimental to achieving high beam quality. This drawback can be overcome by Kerr beam self-cleaning in graded-index MMFs. As pulse power grows above a certain threshold, this process introduced by Kerr nonlinearity allows for an irreversible transfer of power from the high order modes into the fundamental mode, leading to a stable and robust bell-shaped output transverse profile. In this framework, the BEAMLOCKER project aims at studying the dynamics of STML MMF lasers with high-quality output beams by appropriate modeling. The models will be established starting from preliminary experimental studies, and they will be studied by analytical methods, direct numerical simulations, and numerical continuation and bifurcation methods. In addition, the noise behavior of STML lasers will be investigated in detail, with a particular focus on the beam self-cleaning regimes. The project results will stimulate further theoretical analysis and experiments on complex interactions in nonlinear multimode fiber systems. Furthermore, understanding the complex properties of STML MMF lasers will shed new light on frontier physical phenomena, such as spatiotemporal rogue waves generation and turbulence, which find applications in several fields, ranging from telecommunication to biomedicine.